The present invention relates to intake and exhaust passages in cylinder heads of an opposed-cylinder engine.
FIG. 1 schematically shows an opposed six-cylinder engine.
The opposed six-cylinder engine comprises a crankcase A including a pair of cylinder blocks B formed on opposite sides thereof and cylinder head 1 and 1' secured to the cylinder blocks. Three cylinders are formed in each cylinder block.
FIG. 2 shows one of the conventional cylinder heads as viewed from the inside thereof. The cylinder head has combustion chambers 2a for a No. 1 cylinder, 2b for a No. 3 cylinder and 2c for a No. 5 cylinder. Intake valves 3a, 3b and 3c and exhaust valves 4a, 4b and 4c are provided for respective combustion chambers in the cylinder head. A siamese intake passage 6ab is formed in the cylinder head to communicate intake valve ports of the intake valves 3a and 3b with a single opening 5ab. The valve port of the intake valve 3c is communicated by a single passage 6c with an opening 5c. The openings 5ab and 5c are communicated with an intake manifold 10 (FIG. 1). Similarly, valve ports of the exhaust valves 4a and 4b are communicated with an opening 8ab by a siamese exhaust passage 7ab and the port of the exhaust valve 4c is communicated with an opening 8c by a passage 7c.
In the other cylinder head 1', intake and exhaust passages are formed in the same manner as the cylinder head 1. FIG. 3 shows both cylinder heads disposing in adjacent positions for the convenience of explanation. In the cylinder head 1', a combustion chamber 2c' for the No. 2 cylinder, combustion chamber 2b' for the No. 4 cylinder and combustion chamber 2a' for the No. 6 cylinder are formed. Intake passages 6ab' and 6c' and exhaust passages 7ab' and 7c' are arranged in symmetry with respect to an axis for balancing the cylinder heads.
In such an arrangement, since intake valves 3a and 3c are disposed at outermost positions, the length of each intake passage becomes long in order to equalize the length one of the passages with the length of the others. Therefore, the amount of inducted air becomes small, which causes a reduction of combustion efficiency.
On the other hand, the intake strokes of the No. 1 cylinder and No. 3 cylinder overlap with each other as shown in FIG. 6. Since both cylinders are communicated by the siamese intake passage 6ab, a large amount of air is induced into the No. 3 cylinder, which fires later in the firing order than the No. 1 cylinder, by the inertia of the air induced into the No. 1 cylinder. Further, during the overlapping of the exhaust stroke and the intake stroke of the No. 1 cylinder, residual gases in the exhaust passage of the No. 3 cylinder during the exhaust stroke are induced into the No. 1 cylinder. Therefore, combustion in No. 1 is unequalized in idling operation of the engine. When the No. 6 cylinder begins to induce the air after No. 4, the No. 6 cylinder is the same as the No. 1 cylinder in deterioration of combustion.
In addition, in the siamese exhaust passage 7ab, exhaust gases from both cylinders interfere with each other, particularly at wide open throttle, which will result in reduction of the power of the engine.